Constitutive and numerical modeling for the coupled thermal-hydro-mechanical processes in dual-porosity geothermal reservoir

Enhanced geothermal system (EGS) is viewed as one of the most practical methods to explore geothermal energy in hot dry rock (HDR), which can contribute to zero carbon emissions and provide reliable and renewable energy. Modelling the performance and long-term environmental effects of the EGS remains a challenge as the rock is often fractured and experiences complicated multi-physics coupling behaviour. Despite studies on constitutive or numerical modelling of coupled behaviour in deformable dual-porosity media, those developed models often ignore the fully coupled processes in heat transfer and are highly empirical. Based on a non-equilibrium thermodynamics approach, the Mixture-Coupling Theory, this research derives the fully coupled Thermo-Hydro-Mechanical (THM) governing equations for dual-porosity geothermal reservoirs, addressing the interaction between strain, pore/fracture pressure and temperature. The constitutive model is obtained by the analysis of Helmholtz free energy evolution in dual-porosity reservoirs. The proposed model determines the fully coupled evolution of solid stress, both pore and fracture porosity, and solid entropy density. The governing equations can predict the fully coupled THM effect in dual-porosity geothermal reservoirs. Numerical modelling is then performed to study the production performance and coupled THM response in an EGS. The modelling results show that the production temperature is determined by the coupled THM effects. Porosity change is mainly determined by temperature change (accounting for over 87% of the total porosity change) during the extraction. The porosity change accounts for 1.03% and 1.23% of the initial porosity for pore and fracture, separately.